Fet electrode with carbon gate

ABSTRACT

The surface of a gate insulating membrane of an ion-selective field-effect transistor (ISFET) (10) is coated with a carbon thin membrane (4), and the surface of the latter is coated with an electrolytic polmerization membrane (3) of 2,6 xylenol. The ISFET obtained exhibits hydrogen-ion selectivity, little drift, high stability and little response to light. If the surface of the electrolytic polmerization membrane (3) of 2,6 xylenol is coated with another ion-selective membrane or enzyme-active membrane, various ions and the concentration of a biological substrate can be measured.

This application is a continuation of application Ser. NO. 07/362,392,filed July 19, 1989 now abandoned.

TECHNICAL FIELD

This invention relates to a FET electrode and, more particularly, to anion-sensitive FET electrode.

BACKGROUND ART

FET electrodes, long known in the art, utilize the principle of afield-effect transistor (FET). With regard to the structure andoperation of a FET electrode, an impurity is diffused in a p-typesubstrate, which comprises a substrate of a metal oxide/semiconductorinsulative membrane (p-type SiO₂ /Si₃ N₄), on the gate portion thereof,thereby forming an n-type source and a drain electrode. When a positivevoltage is applied to the electrode at the gate portion, the potentialof the p-type semiconductor in the vicinity of a redox membrane drops toinduce electrons within the p-type semiconductor. A layer of theseelectrons forms a channel along which electrons flow from the source tothe drain to produce a drain current. The amount of this drain currentis controlled by the gate voltage. Since the voltage at the gate isproportional to the H⁺ ion activity, the FET electrode can be used as apH-MOSFET

However, a FET electrode of this type responds to light, besidesexhibiting a large amount of drift and poor stability.

DISCLOSURE OF THE INVENTION

An object of the present invention is to solve the foregoing problems ofthe prior art and provide a FET electrode exhibiting little drift, highstability and little response to light.

As means for solving the foregoing problems, the FET electrode of thepresent invention comprises a FET, a carbon thin membrane coating a gateinsulator of the FET, and an organic thin membrane coating the carbonthin membrane.

In the arrangement of the invention, the FET measures the concentrationof H⁺ ion based on a potential, which corresponds to the activity of theH⁺ ion, produced on the gate portion by the organic thin membrane.Meanwhile, the carbon thin membrane reduces drift, stabilizes theadhesion between the gate insulator of the FET and the organic thinmembrane and shuts out light.

Thus, in accordance with the invention, there is provided a FETelectrode exhibiting little drift, high stability and little response tolight.

Moreover, since the membrane covering the gate produces a potentialwhich corresponds to the H⁺ ion, there can be provided a FET electrodewhich operates on the principle of the field effect, namely a FETelectrode having the following structural arrangement, which ischaracteristic of the field effect:

(1) An amplifier having an high input impedence is unnecessary.

(2) Since a negative feedback circuit is constructed by utilizing theamplifying action of the device, the output resistance of the electrodecan be kept to a low several thousand kilohms and electricaldisturbances can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view illustrating an integrated-type FET electrodeof the present embodiment;

FIG. 2 is a view for describing an apparatus for measuring the FETelectrode of the present embodiment;

FIG. 3 is a view illustrating the results obtained by measuring the FETelectrode of the present embodiment; and

FIG. 4 is a perspective view of an isolated-type FET electrode.

BEST MODE FOR CARRYING OUT THE INVENTION

First, examples of forming a carbon thin membrane will be illustrated.

Formation Example 1

By using carbon (high-purity graphite carbon G161AS, manufactured byTokai Carbon K.K.) as a target, a carbon thin membrane was deposited onthe surface of sapphire (on silicon) by a sputtering process.

The conditions for sputtering were 100 W, 8×10⁻² Torr, 20 hrs, asubstrate temperature of less than 150° C. and an argon atmosphere.

As a result, there was obtained a carbon-coated sapphire substratehaving a carbon thin membrane thickness of about 1.0 μm.

Formation Example 2

The sputtering conditions were the same as those in Experiment 1 exceptfor the fact that a methane gas atmosphere was used.

As a result, there was obtained a carbon-coated sapphire substratehaving a carbon thin membrane thickness of about 1.2 μm. A strongmembrane could be produced, and the specific resistance was 1×10⁻³ Ωcm.

Formation Example 3

The sputtering conditions were the same as those in Formation Example 1except for the fact that a hydrogen gas atmosphere was used.

As a result, there was obtained a carbon-coated sapphire substratehaving a carbon thin membrane thickness of about 0.8 μm. The specificresistance was 1 ×10⁻³ Ωcm.

Comparison Formation Example

As in Formation Example 1, carbon was used as the target to coat thesurface of sapphire (on silicon) with a carbon thin membrane by asputtering process.

The conditions for sputtering were 600 W, 1×10⁻² Torr, 20 min, asubstrate temperature of 300° C. and an argon atmosphere.

As a result, there was obtained a carbon-coated sapphire substratehaving a carbon thin membrane thickness of about 1000 Å.

An electrode and a FET electrode using the formed carbon-coated sapphiresubstrate will now be described.

EXAMPLE 1

The specific resistance of the carbon-coated sapphire substrate (1 cm×1cm in size) obtained in Formation Example 1 was about 10⁻³ Ωcm. Theperiphery of the substrate was insulated with a silicone resin (KE348W,manufactured by Shinetsu Silicone K.K.). A silver coaxial line (0.6 mmφin size) was attached to one side by means of an electrically conductiveadhesive to form a lead wire. The resulting electrode (active electrode)had a response area of about 0.5 mm×0.5 mm at its tip.

The surface of the electrode was coated with a polymeric membrane of 2,6xylenol by an electrolytic polymerization process carried out under thefollowing conditions:

Composition of electrolyte solution

0.5M 2,6 xylenol

0.2M NaClO

acetonitrile solution (solvent)

ELECTROLYTIC POLYMERIZATION CONDITIONS

The electrolyzing potential was swept three times (sweep rate: 50mV/sec) from 0 to +1.5V (vs. SSCE), followed by carrying outconstant-potential electrolysis for 10 min at a constant potential of+1.5V.

Experiment 1

Using the redox membrane-coated carbon-sapphire electrode fabricated inExample 1, the relationship between the electromotive force producedacross this electrode and a reference electrode (an Ag/AgCl electrode)and a change in pH in a phosphate buffer solution was determined. As aresult, a linear

relationship was found over a wide pH range of pH 1.0-9.0, and the slopeof the straight line was 58-59 mV/pH (25° C.), thus substantiallyapproximating the Nernst theoretical equation.

The speed of response was substantially the same as that of a redoxmembrane-coated carbon electrode (coated wire-type electrode), namely5-30 sec (pH range of pH 5 -9).

Thus, a semiconductor substrate (silicon or sapphire) could be coatedwith a stable carbon thin membrane

EXAMPLES 2 AND 3

Redox membrane-coated, carbon-membrane silicon substrate electrodes werefabricated under the same conditions as set forth in Example 1.

Experiments 2 and 3

A change in pH with respect to the emf developed by the electrodesprepared in Examples 2 and 3 was measured as in Experiment 1. As aresult, a linear relationship was found over a wide pH range of pH1.0-9.0, and the slope of the straight line was 58 mV/pH (25° C.), thussubstantially approximating the Nernst theoretical equation. The speedof response was a quick 5-30 sec.

Example 4

As shown in FIG. 1, a FET electrode 10 was prepared by coating the gateinsulator of a MOSFET with a carbon thin membrane, and coating thelatter with a polymeric membrane of 2,6 xylenol (set forth in Example 1)as an organic thin membrane by means of an electrolytic polymerizationprocess. Numeral 1 denotes a drain, 2 a source, 3 the polymeric membraneof 2,6 xylenol, 4 the carbon thin membrane, 5 a silicon nitridemembrane, 6 a silicon oxide membrane, 7 p-type silicon, and 8 sapphire

The emf developed by the FET electrode 10 thus prepared was measuredwith respect to the pH of a liquid specimen 12 using the measurementapparatus shown in FIG. 2. Numeral 11 denotes a reference electrode, 13a measurement circuit, and 14 a digital voltmeter. Measurement wasperformed under the following conditions: I_(s) =100 μA, V_(DS) =4V,temperature=25° C.

As indicated by the results shown in FIG. 3, the slope of V_(OUT) /pH(=-58.3 mV/pH) is a straight line. This well approximates the Nernsttheoretical equation. The speed of response obtained was 5-30 sec. TheMOSFET characteristics (I_(D) -V_(GS) and I_(D) -V_(DS)) of the FETsensors where the characteristics peculiar to the respective FET's.

Though the present embodiment has been described with regard to theintegrated-type FET electrode shown in FIG. 1, the same results can beobtained even with an isolated gate-type FET electrode of the kind shownin FIG. 4. The same results can also be obtained using a p-type siliconsubstrate or an n-type silicon substrate as the substrate. Preferably,the specific resistance of the carbon-coated sapphire substrate is lessthan 10 Ωcm, particularly less than 1 Ωcm and, most preferred over all,less than 1×10⁻³ Ωcm, as illustrated in the present embodiment.

The FET electrode can be used as a biosensor, such as an ion-selectiveFET sensor, a gas sensor for oxygen or the like and an enzyme sensor, bycoating the the organic thin membrane of the FET electrode of thepresent embodiment with an ion-carrier membrane (a neutral carriermembrane), an oxygen-active membrane or an enzyme-fixed membrane, etc.

What is claimed is:
 1. A FET device comprising:a FET having a source, adrain, a channel for linking between the source and the drain and a gatefor controlling current flow in the channel by field effect, aninsulator over said channel of said FET, a carbon membrane coating saidinsulator and forming said gate, and an organic membrane formed on asurface of said carbon membrane by an electrolytic oxidativepolymerization process.
 2. The FET device according to claim 1, whereinsaid carbon membrane possesses, in part, a multi-surfaced structure, andthe specific resistance is less than 10 Ωcm following application of thecarbon membrane.
 3. The FET device according to claim 1, wherein saidorganic membrane is a membrane which manifests an oxidation-reductionresponse.
 4. The FET device according to claim 1, further comprising andion-carrier membrane coating said organic membrane.
 5. The FET deviceaccording to claim 1, further comprising an oxygen-active membranecoating said organic membrane.
 6. The FET device according to claim 1,wherein said carbon membrane is further extended from said FET, and saidorganic membrane is formed on a surface of said extended carbonmembrane.
 7. A FET device comprisinga FET having a source, a drain, achannel for linking between the source and the drain and a gate forcontrolling current flow in the channel by field effect, one of thesource and the drain surrounding the other; an insulator over saidchannel of said FET; a carbon membrane coating a portion of saidinsulator far from both electrodes of the source and the drain andforming said gate; and an organic membrane formed on a surface of saidcarbon membrane by an electrolytic oxidative polymerization process. 8.The FET device of claim 7, wherein said carbon membrane possesses, inpart, a multi-surfaces structure, and the specific resistance is lessthan 10 Ωcm following application of the carbon membrane.
 9. The FETdevice of claim 7, wherein said organic membrane is a membrane whichmanifests an oxidation-reduction response.
 10. The FET device of claim7, further comprising an ion-carrier membrane coating said organicmembrane.
 11. The FET device of claim 7, further comprising anoxygen-active membrane coating said organic membrane.